Modern communications networks generally carry two types of traffic or data. The first is the traffic which is transmitted by or delivered to a user or subscriber, and which is usually paid for by the user. That type of traffic is widely known as user traffic or subscriber traffic. The second is the traffic caused by network management applications in sending and receiving management data from network elements, known as management traffic.
In telecommunications, the management traffic is also known as signaling traffic. The term “signaling” refers to the exchange of signaling messages between various network elements such as database servers, local exchanges, transit exchanges and user terminals. A well known protocol for transferring such signaling messages is the Signaling System 7 (SS7), also referred to as Common Channel Signaling System 7 (CCS7).
The Signaling System 7 as specified by the International Telecommunication Union (ITU) in the Q.700-series standards provides for all signaling tasks in today's telecommunications networks. More specifically, SS7 provides for example for:                basic call setup, management, and tear down;        enhanced call features such as call forwarding, calling party name/number display, and three-way calling;        accounting and billing;        database operations for services such as authentication, roaming, local number portability (LNP), toll-free services and special tariff services;        network management for the SS7 network and its connections; and        non-call related signaling, allowing for services such as short message service (SMS) and user-to-user signaling (UUS).        
The Signaling System 7 forms an independent network, in which SS7 messages are exchanged between network elements over bidirectional channels called signaling links. Signaling occurs out-of-band on dedicated channels rather than in-band on channels reserved for user traffic such as voice. Compared to in-band signaling, out-of-band signaling provides:                faster call setup times;        more efficient use of voice circuits; and        support for Intelligent Network (IN) services which require signaling to network elements without voice trunks (e.g., database systems).        
The elements of a SS7 network are known as signaling points, each uniquely identified by a signaling point code. Point codes are carried in signaling messages exchanged between signaling points to identify the source and destination of each message. Each signaling point uses a routing table to select the appropriate signaling path for each message.
There are three kinds of signaling points in a SS7 network: Service Switching Points (SSPs), Signaling Transfer Points (STPs), and Service Control Points (SCPs)
SSPs are switches that originate, terminate, or tandem calls. An SSP sends signaling messages to other SSPs to setup, manage, and release voice circuits required to complete a call. An SSP may also send a query message to a centralized database (an SCP) to determine how to route a call (e.g., a toll-free call). An SCP sends a response to the originating SSP containing the routing number(s) associated with the dialed number.
Network traffic between signaling points may be routed via signaling transfer points (STPs). An STP routes each incoming message to an outgoing signaling link based on routing information contained in the SS7 message. Because it acts as a network hub, an STP provides improved utilization of the SS7 network by eliminating the need for direct links between signaling points. An STP may perform global title translation, a procedure by which the destination signaling point is determined from digits present in the signaling message (e.g., the dialed 800 number, calling card number, or mobile subscriber identification number).
The SS7 uses a protocol stack, in which the hardware and software functions of the SS7 protocol are divided into functional abstractions called “levels”. These levels map loosely to the Open Systems Interconnect (OSI) 7-layer model defined by the International Standards Organization (ISO).
The lower three levels are known as the Message Transfer Part (MTP). MTP Level 1 defines the physical, electrical, and functional characteristics of the digital signaling link. MTP Level 2 ensures accurate end-to-end transmission of a message across a signaling link. MTP Level 3 provides message routing between signaling points in the SS7 network.
In SS7, functions are provided by so called user parts. A widely used user part is the ISDN User Part (ISUP) which defines the protocol used to set-up, manage, and release trunk circuits that carry user traffic between terminating line exchanges (e.g., between a calling party and a called party). In some countries, the less sophisticated Telephone User Part (TUP) performs these tasks.
Another protocol in SS7, the Signaling Connection Control Part (SCCP), provides connectionless and connection-oriented network services and global title translation (GTT) capabilities above MTP Level 3. SCCP is used as the transport layer for TCAP-based services.
The Transaction Capabilities Applications Part (TCAP) supports the exchange of non-circuit related data between applications across the SS7 network using the SCCP connectionless service. Queries and responses sent between SSPs and SCPs are carried in TCAP messages. In mobile networks, TCAP carries Mobile Application Part (MAP) messages sent between mobile switches and databases to support user authentication, equipment identification, and roaming.
Problematically, congestion control methods as implemented in previous SCCP stacks only provide for traffic limitation if either the bandwidth of a link or the processing capability at a destination is exceeded. With such congestion control methods, high-bandwidth traffic associated with one service may severely affect other, more important services.
Furthermore, the signaling traffic transferred in a signaling network is subject to a complex set of accounting and billing rules leading to well-compensated traffic types and less profitable signaling traffic. Some traffic types are necessary to provide control information for optimal network performance. As a consequence, network operators wish to assign priorities to the various types of signaling traffic accordingly to ensure optimal network performance and to achieve a profitable balance between well-compensated traffic types and non-compensated traffic types.